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SOFIA, the two-fold stargazer

Credit: DLR / Clemens Plank

I have worked in the Space Science Department at the DLR Space Administration since October 2016. I could not imagine a better job than my own as SOFIA project engineer. I tend to commute between Bonn and California quite regularly as the NASA/DLR airborne observatory has its home base in Palmdale. But for around a month now, I have been in Hamburg with SOFIA.

Like any other aircraft, this one – registered as N747NA and named the Clipper Lindbergh – requires regular maintenance. This is no ordinary aircraft, though, as the Clipper Lindbergh is just one of 45 special variants of the classic Boeing 747 that were ever built: a so-called 747SP (special performance). Fewer than eight of these 'old timers' remain in service today. What is more, the Clipper Lindberg was modified to make space for the SOFIA telescope, which weighs 'just' 17 tons. So we are dealing with quite a few special features all at once. That is why inspections for its next certification are now on the agenda, and they are far more detailed than those for normal airliners or cars.##markend##

So it is handy that there is a 'specialist aircraft workshop' in Hamburg that is extremely familiar with the 747SP type. An even greater advantage is that this workshop regularly services 'special mission' aircraft.

Unusual and by no means everyday situations crop up repeatedly in this kind of exhaustive and out-of-the-ordinary inspection. As photography is my favourite hobby, in addition to astronomy, I will let the pictures speak for themselves.

Credit: DLR / Clemens Plank

SOFIA touched down in Hamburg on 20 November 2017 at 08:45 local time

It was taken immediately for the first functional tests in order to assess SOFIA's 'general state of health'. These tests were carried out in an open hangar at the Lufthansa Technik complex.

However fascinating SOFIA's intricate engineering may be, the science it conducts is even more fascinating

An approximately six-by-four metre sliding door was fitted to the fuselage of the aircraft between 1997 and 2007 in order to give the infrared telescope an unobstructed view of the stars. But few people know that even the 'original model' of the 747SP already allowed celestial observation through a sliding door. Although this door was significantly smaller and designed for a different purpose, it was nevertheless almost as exciting as the current feature.

This original sliding door or hatch is still located at the top of the cockpit, behind the seat of the flight engineer, whose duties in the past included using a sextant to double check the imprecise GPS position during transatlantic flights. This is the same technology that seafarers used in bygone times to navigate the 'eternal ocean'. Interestingly: it is not just sailors that use a sextant; Apollo astronauts also used one to adjust their trajectory to the Moon.

This method of navigation is now a thing of the past, of course, and is no longer used in aircraft. But it was once absolutely crucial on long-haul flights, as the dead zone above the Atlantic and the declination of the magnetic needle on the compass close to the pole presented severe challenges for pilots. This small sliding door was therefore of invaluable assistance.

Credit: DLR / Clemens Plank

Left: the approximately five-centimetre sliding door, closed
Right: the opened hatch offers a clear view of the stars during flight

The sextant is fitted on the metal mount around the opening. The sliding door is opened by simply pulling on the yellow knob. This will significantly increase the volume level during flight, but it does not lead to any dangerous pressure fluctuations in the cabin, as the opening is too small. The scarf in the next images indicates where SOFIA's 'original sliding door' is located.

Credit: DLR / Clemens Plank

It is not always an 'X' that marks the spot. In this image, it is a scarf that sinks into the opening of the sextant. The much larger hatch in the foreground is the emergency exit.

Meticulous maintenance – scrutinising SOFIA

This kind of hatch is nowhere to be found in modern aircraft, of course. But even the latest models require regular maintenance. One of the most fascinating aspects during maintenance is the docking procedure. It takes place after the initial functional tests. Docking means that the aircraft is mounted on supports, known as jacks, and then surrounded by scaffolding.

Credit: DLR / Clemens Plank

The 'docked' aircraft, surrounded by walkable scaffolding

This gives the maintenance staff easier access to all areas of the aircraft. The approximately 300-ton 747SP is positioned on five jacks in order to conduct thorough inspections of the undercarriage as well.

Credit: DLR / Clemens Plank

The tripods marked with red arrows are known as jacks

Padding, a version of jacking

Jacking means that the weight is transferred from the landing gear, where it normally rests, to five defined attachment points on the aircraft. This is the best way to provide access for work on the undercarriage. But jacking also causes a slight deformation of the fuselage, as the aircraft 'bends' due to the unnatural load distribution. Although not severe, the change in shape is visible – and can prevent other maintenance work on the aircraft. This is why SOFIA was additionally placed on so-called pads, which spreads the weight of the aircraft quite naturally across the landing gear.

Credit: DLR / Clemens Plank

A rare view of the aircraft from below: the landing gear stands on pads; sections of the fuselage are open, and some tyres on the main landing gear have been removed (on the left).

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Clemens Plank studied mechanical engineering with a focus on aerospace engineering and nuclear technology at the Technical Univeristy of Munich (TU Munich). He completed his dissertation at the Massachusetts Institute of Technology (MIT) in the United States, in which he investigated computer simulation of hydrogen deflagration.
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